The invention relates to a storage catalyst.
WO 97/02886, which forms the basis of the generic introduction, has disclosed a storage catalyst for reduction of nitrogen oxides (NOx) in exhaust gases from lean-mix engines, which contains a component which stores NOx and a component which catalytically reduces NOx. The catalytically active component and the storing component are applied to a support body at least substantially separately from one another. The component which catalytically reduces the NOx is at least one element from the platinum group which is arranged separately from the component which stores the NOx. The component which stores the NOx comprises one or more materials from the group consisting of metal oxides, metal hydroxides, metal carbonates and mixed metal oxides, the appropriate metal in each case being lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium and/or barium.
In operation, particularly in the presence of oxygen, such storage catalystsxe2x80x94which preferably contain platinum and a barium compoundxe2x80x94are subjected to high thermal loads and, especially at temperatures above 600xc2x0 C., experience rapid ageing. However, such conditions are experienced during the release of the NOx which has previously been held in place and during the regeneration of sulphate poisoning, and consequently known storage catalysts have a relatively short service life.
A further storage catalyst for lean-mix engines is known, for example, from EP 562,805 A1. This storage catalyst is a transition metal/zeolite catalyst in which the transition metal has been introduced into the zeolite support body by means of ion exchange. Furthermore, this document has disclosed an exhaust system for lean-mix engines, and an engine control unit which is required in order to reduce the level of pollutants.
The object of the invention is to further develop the storage catalyst on which the generic introduction is based in such a manner that its thermal ageing is reduced. It is intended that the storage catalyst should exhibit this reduced ageing in particular when used in exhaust pipes of internal-combustion engines which are operated in a mixed lean-burn/rich mode, and particularly preferably during temperature adjustment.
The object is achieved, with a storage catalyst according to the present invention. Due to the novel combination of the catalytically active component and the NOx-storing component with a low level of chemical activity with respect to one another at elevated temperatures (higher than 600xc2x0 C., in particular higher than 800xc2x0 C.), the storage catalyst according to the invention has a long service life even at these temperatures. Furthermore, it is also less expensive to produce, partly because precious metals are at least substantially eliminated.
Useful refinements to the invention can be found in the subclaims. Furthermore, the invention is explained in more detail with reference to exemplary embodiments which are illustrated in the figures, in which: